Functional analysis of a tomato (Solanum lycopersicum L.) rhamnogalacturonan lyase promoter

The enzyme rhamnogalacturonan lyase (RGL) cleaves α-1,4 glycosidic bonds located between rhamnose and galacturonic acid residues in the main chain of rhamnogalacturonan-I (RG-I), a component of the plant cell wall polymer pectin. Although the mode of action of RGL is well known, its physiological functions associated with fruit biology are less understood. Here, we generated transgenic tomato plants expressing the β-glucuronidase (GUS) reporter gene under the control of a -504 bp or a -776 bp fragment of the promoter of a tomato RGL gene, Solyc11g011300. GUS enzymatic activity and the expression levels of GUS and Solyc11g011300 were measured in a range of organs and fruit developmental stages. GUS staining was undetectable in leaves and roots, but high GUS enzymatic activity was detected in flowers and red ripe (RR) fruits. Maximal expression levels of Solyc11g011300 were detected at the RR developmental stage. GUS activity was 5-fold higher in flowers expressing GUS driven by the -504 bp RGL promoter fragment (RGFL3::GUS) than in the isogenic line, and 1.7-fold higher when GUS gene was driven by the -776 bp RGL promoter fragment (RGLF2::GUS) or the constitutive CaMV35S promoter. Quantitative real-time polymerase chain reaction analysis showed that the highest expression of GUS was in fruits at 40 days after anthesis, for both promoter fragments. The promoter of Solyc11g011300 is predicted to contain cis-acting elements, and to be active in pollen grains, pollen tubes, flowers and during tomato fruit ripening, suggesting that the Solyc11g011300 promoter is transcriptionally active and organ-specific.